Such a capacitive sensor assembly comprises at least one capacitive sensor element, which is formed by at least one (first) electrical conductor element as electrode and which is connected or to be connected to an evaluation unit, which determines and evaluates at least one measured value in dependency on the capacitive coupling of the capacitive sensor element to its surrounding, as well as at least one electrical heating element, which is formed by at least one electrically operated heating conductor and which is being spaced apart from the capacitive sensor element, thus in particular from the first electrical conductor element. The term conductor element is thereby in general being used for one or multi-part electrical conductive element, which can be for instance provided as a cable, flat conductor, electrical conductive film or in any form as an electrical conductive component (electrical conductive assembly).
The assigned evaluation unit can determine and evaluate in particular the change of a measured value (for instance a measured current) in dependency on the capacitive coupling of the capacitive sensor element to a reference potential, which can be defined by a suitable vehicle element, as for instance components of the vehicle body, components of a vehicle seat or the mentioned electrical heating element, wherein this capacitive coupling and thus said measured value is being influenced by the presence of a vehicle occupant or any other object in the surrounding of the capacitive sensor element. In case of a capacitive sensor element being integrated in a vehicle seat, for instance the presence of a vehicle occupant being located on the seat can be herewith determined and also the occupant's condition can be concluded or a child seat can be recognized.
It is known from U.S. Pat. No. 6,563,231 B1 to provide at least one electrical conductor element as electrode on a sensor mat for forming a capacitive sensor element. The sensor mat is arranged as an occupant detector on a vehicle part in order to determine the occupation of a specific seat of a motor vehicle with a vehicle occupant. Specifically, the sensor mat is being arranged with the electrical conductor element serving as a capacitive sensor element for instance on a seat area of a motor vehicle seat, and namely in particular between the seat cover and a seat cushion, thus approximately below the seat cover and above the seat cushion. In order to determine, if a vehicle occupant is located on a vehicle seat being equipped with such a detector, the capacity of the sensor mat and a carrier of a reference potential (zero potential) for instance defined by the vehicle floor (“earth”) or by at least one reference conductor element integrated in the sensor mat is determined. This does have by no means necessarily to occur directly by a (direct) capacity measurement; rather it can occur by the measurement of any electrical values, which are linked to the mentioned capacity, wherein the respective measured value thereof depends on the capacity (between the sensor mat and a carrier of the reference potential), as for instance by the measurement of measured currents (in or to the electrical conductor element serving as capacitive sensor element), compare also DE 100 48 956 C1 in case of a capacitive sensor assembly being integrated into a steering wheel.
If a vehicle occupant is located on the corresponding vehicle seat then the resulting capacity is being influenced by the capacitive coupling of the occupant; and the resulting capacity corresponds to a combination (depending on the specific conditions for instance approximately a serial connection) of the capacity between the seat surface (or the capacitive sensor element provided there) and the vehicle occupant as well as the capacity between the vehicle occupant and the reference potential. By integrating thereby a reference conductor element with reference potential as counter electrode into the vehicle seat or into the sensor mat itself the distance between the carrier of the reference potential and the vehicle occupant to be detected can be reduced and through this the measuring sensitivity can be improved.
With such an arrangement it can also be differentiated if a vehicle occupant is located directly on the corresponding vehicle seat or if a child seat is provided on the vehicle seat, which in turn can accommodate a toddler. This assessment can be used in order to optionally deactivate one or more of the airbags assigned to the corresponding seat for instance when none-occupied or occupied with a child seat. Thus, a capacitive sensor assembly can be used as an occupant detector in order to activate or deactivate safety devices of a motor vehicle, but also for triggering other functions of a motor vehicle as for instance warning or controlling functions.
It is furthermore known from practice to arrange additionally an electrically operated heating conductors on a sensor mat beside the at least one electrical conductor element serving as a sensor element, wherein said heating conductors are being spaced apart from the first electrical conductor element and which produce under current heat for heating a steering wheel, a vehicle seat or any other vehicle part.
In case of capacitive sensor systems, which comprise beside a capacitive sensor element also at least one electrical heating element and/or at least one reference conductor element, the compliance of a defined distance between the (first) electrical conductor element forming the capacitive sensor element as well as the heating conductor forming the electrical heating element or the respective reference conductor element is of importance in order to be able to obtain reliable statements about the occupation of a seat with a vehicle occupant. The effect of humidity and moisture on the measured results is also in particular dependent on the distance between the sensor element and heating element or sensor element and reference conductor element,